An ultra-wideband orthogonal-beam directional graphene-based antenna for THz wireless systems

In terahertz (THz) wireless systems, graphene shows a tremendous promise for the implementation of miniaturized and reconfigurable antennas due to its unique tunable property. This paper presents a wideband beam reconfigurable directional antenna for THz wireless communication systems. The antenna design concept is based on the Yagi-Uda antenna working principle. The proposed antenna consists of a centre-fed graphene-based driven dipole and three graphene-based parasitic elements on either side of the driven element. These graphene-based parasitic elements either act as directors or reflectors by controlling the surface conductivity of these elements. The surface conductivity of the elements is adjusted individually by applying a bias voltage via the chemical potential of the graphene. The main beam direction of the antenna can be reconfigured by controlling the chemical potentials of the graphene-based parasitic elements. Specifically, the proposed graphene THz antenna reconfigures the main beam into four orthogonal directions (0°, 90°, 180° and 270°) at an operational frequency 1.25 THz. The antenna achieves a directional symmetrical radiation pattern with 14 dBi gain and a front-to-back ratio of 15.5 dB. Furthermore, the proposed graphene THz antenna provides a stable reflection coefficient in four reconfigurable cases and an ultra-wideband of 10-dB impedance bandwidth of 120%. Therefore, this novel design technique of graphene-based ultra-wideband high gain beam reconfigurable THz antenna is promising for THz wireless systems.

www.nature.com/scientificreports/ complicated structure for a reconfigurable four-beam antenna using the Yagi-Uda antenna concept with metal as radiator and graphene as switches for reconfiguration. A design for a switched beam graphene THz antenna using Yagi-Uda concept has been provided in 25 . In this work, each parasitic element consists of three different graphene strips and are separated by a small distance of 500 nm, which increases the complexity in the antenna structure. The conductivity of graphene strips of each parasitic element adjusted to make the overall length of the parasitic element larger/smaller than the length of the excited dipole, and hence act as a reflector/director of the antenna structure. The bandwidth information of these beam reconfigurable antennas is not available in most of the literature. However, the 10-dB impedance bandwidth of the four-beam reconfigurable antenna in 24 and 25 are 10% and 42%, respectively. In 26 , multi-beam reconfigurable antenna based on graphene at 1.243 THz achieved 10-dB impedance bandwidth of 10.5%, and the gain of 6.5 dB with a 12.1 dB front to-back ratio. A beam-scanning seven-element quasi-Yagi-Uda antenna based on hybrid metal-graphene materials is deflected from −7 0 • to + 70 • by controlling the bias voltage of graphene in 27 . An array of graphene-based Yagi-Uda antennas is reported in 28 , which consists of four identical Yagi-Uda antennas, reconfigures the radiation beam from − 75 • to + 75 • with directivity of 9.78 dBi at the 2.5 THz resonant frequency with 12.38% operating wide bandwidth. In comparison to previous works, in this report, we present a novel and simple way to design a graphene Yagi-Uda antenna with wideband and beam reconfiguration capability in the THz band. In this work, each parasitic element is a single graphene strip of dynamic conductivity, which makes its implementation simpler than previous works. Moreover, the proposed antenna provides wide bandwidth of 120% and high directivity of 14 dBi compared to previous works. The antenna is capable of reconfiguring its radiation pattern, covering a 360 • angle. The main beam direction of the graphene THz antenna is achieved by controlling the chemical potentials of the parasitic elements. In particular, the main contributions of the paper are summarized as follows.
• We propose a simple structured wideband beam reconfigurable THz antenna using novel graphene nanomaterial. The antenna design concept is based on the Yagi-Uda antenna principle. The antenna consists of graphene-based driven dipole and parasitic elements. • We design a graphene-based Yagi-Uda reconfigurable antenna by using the electromagnetic (EM) simulator CST Studio Suite and provide the simulation technique for the EM modelling of graphene elements and the beam reconfiguration for the proposed antenna structure. • We investigate the performance of the proposed antenna by controlling the chemical potential of the graphene-based driven and parasitic elements. The antenna's radiation direction is reconfigurable, covering a 360 • angle with four orthogonal beams ( 0 • , 90 • , 180 • and 270 • ) at an operational frequency of 1.25 THz. Moreover, the antenna provides ultra wideband of bandwidth about 120% , the gain of 14 dBi and a front to back ratio of 15.5 dB. • Furthermore, the experimental feasibility and the required bias voltage to enable the beam reconfiguration of the proposed antenna is discussed.

Results
To evaluate the proposed graphene-based THz antenna structure, it is essential to model the conductivity of graphene at THz frequencies. According to Kubo formalism, the surface conductivity of graphene in the THz band can be approximated as 29 where j is the imaginary unit, K B is the Boltzmann's constant, e is the electronic charge, T is the temperature, ω is the angular frequency, is the reduced Planck's constant, τ is the relaxation time and µ c is the chemical potential. The conductivity of graphene is strongly dependent on the chemical potential µ c and can be tuned by controlling the chemical potential µ c level. Specifically, the chemical potential µ c can be dynamically adjusted using electric field effect by means of chemical doping or DC bias voltage, thereby tuning the graphene conductivity. An applied electric field bias injects more electron or hole carriers, which allows to dynamically control the complex conductivity. The applied electric field E can be approximated as 30 .
Moreover, the chemical potential parameter of graphene can be adjusted by applying a bias voltage to the graphene layer, which can be approximated as 31 where C = ε r ε 0 /t is the electrostatic capacitance, ε r is the relative permittivity of the substrate, ε 0 is the absolute permittivity in free space, t is the thickness of the substrate, and v f is the Fermi velocity of the graphene.
The bias voltage V g can be approximated from (3) as In the design of the proposed antenna, the chemical potential µ c of 0.4 eV, 0.6 eV and 0.8 eV are considered to achieve the director, driven and reflector state. The changing of chemical potential parameter enables the reconfiguration in the graphene THz antenna. Experimentally, the variation of chemical potential is achieved by applying different bias voltages to graphene strips 32 . For instance, in the present case, the chemical potential µ c = 0.4 eV is achieved with a bias voltage V b = 1.81 V, generating a static electric field E = 9.05 kV/cm. With the increase in the applied bias voltage, more charge is induced on the graphene element, which in turn increases the chemical potential. The tunable chemical potential of graphene enables the different states and reconfigurations in the graphene-based THz antennas. In the present work, the tunable conductivity of graphene via chemical potential is used to reconfigure the main beam into different directions.
A graphene-based Yagi-Uda reconfigurable antenna with four orthogonal beams at THz band is investigated. The design of the proposed antennas is based on the principles of the Yagi-Uda antenna. The three-dimensional (3D) view, cross-sectional view, and top view of the proposed graphene-based Yagi-Uda antenna are shown in Fig. 1a, b, and c, respectively. The antenna consists of a centre-fed graphene-based driven dipole and three graphene-based parasitic elements on either side of the driven element. These graphene-based parasitic elements act as reflectors or directors depending on the chemical potential of the graphene. All graphene antenna elements are placed over a metallic grounded silica ( SiO 2 ) substrate of circular geometry. The radius and height of the substrate are 100 µm and 2 µm, respectively. The platinum metal is used as a ground plane for the graphene THz antenna structure. The dimensions of the antenna are optimized for 1.25 THz operational frequency. For this frequency of the antenna, the values of geometrical dimensions of antenna elements are given in Table 1. Due to the propagation of SPP waves in graphene at THz, the proposed graphene THz antenna resonates at a sub-wavelength  In the proposed graphene-based Yagi-Uda THz antenna, the graphene-based parasitic elements either act as directors or reflectors by controlling the surface conductivity of these elements. The surface conductivity of the elements is adjusted individually by applying a bias voltage via the chemical potential of the graphene. The chemical potential of the graphene strip determines its conductivity and hence its active charge concentration region or electrical length. The reflectors are the graphene parasitic elements with higher chemical potential of graphene strip, which means a larger electrical length. The directors are the graphene parasitic elements with smaller chemical potential, which means a smaller electrical length. The dependency of graphene conductivity on the chemical potential parameter enables the reconfiguration in the graphene THz antenna. In the present work, the chemical potential µ c values for the driven dipole, director and reflector are 0.6, 0.4 and 0.8 eV, respectively, which are listed in Table 2. If the chemical potential of graphene-based parasitic element is µ c = 0, means no bias is applied to graphene-based element. The required active charge concentration region or electrical length to become reflector, director or driven dipole element is not fulfilled. Henceforth, the graphene based element with µ c = 0 neither act as a reflector, director nor driven. Table 3 shows the dependence of chemical potential on bias voltage for the proposed antenna structure. From Table 3, it can be noticed that the bias voltage of 1.8 V, 2.7 V, and 3.6 V are required to enable the beam reconfiguration of the proposed antenna structure.
The proposed THz antenna consists of two graphene-based driven dipole elements (1 and 8) at the middle and 12 graphene-based parasitic elements on either side of the driven element. In addition, 2, 3, and 4 are three parasitic elements on top; 5, 6, and 7 are three parasitic elements on bottom; 9, 10, and 11 are three parasitic elements on left side; 12, 13, and 14 are three parasitic elements on right side. Feeding has been provided at the centre of driven elements 1 and 8. Two driven dipole elements 1 and 8 are placed in crossed manner at the middle, whereas as three parasitic elements are placed with equidistant in top, bottom, left and right side . The proposed graphene THz antenna operates in four different modes and provides four orthogonal beams. For each working mode, one driven element, one reflector and three directors are required. In mode D1, the element 1 is the driven dipole, elements 2, 3, and 4 are directors and the element 5 is the reflector. Similarly, the element 8 acts as the driven, the element 12 as the reflector and elements 9, 10, and 11 as the directors for mode D2; the element 1 as the driven, the element 2 as the reflector and elements 5, 6, and 7 as the directors for mode D3; the element 8 as the driven, the element 9 as reflector and elements 12, 13, and 14 as the directors for mode D4. The four modes with the chemical potential values of graphene antenna elements are listed in Table 4.
The graphene properties and the dimensions are chosen for the antenna to operate at 1.25 THz frequency. The S 11 parameter of the antenna in four working modes D 1 , D 2 , D 3 and D 4 are shown in Fig. 2. Due to the plasmonic wave propagation in graphene at THz, the proposed graphene THz antenna resonates at a sub-wavelength scale. At 1.25 THz frequency, the SPP wavelength SPP = 0 /2.5 , where 0 is the wavelength of free space. Moreover, the antenna provides an ultra wideband of 10-dB impedance bandwidth of 120% . The tunable graphene conductivity makes it possible to allow four different operation states. Using this unique tunable property, the reconfigurable antenna with four directional beams is presented in Fig. 3. The surface current distributions and the 3D far-field radiation pattern of the proposed antenna in mode D 1 are presented in Fig. 4. The antenna attains a unidirectional symmetrical radiation pattern with a gain of 14 dBi. The gain and radiation efficiency of the proposed graphenebased Yagi-Uda THz antenna over the frequency band 0-4 THz are shown in Fig. 5. It can be seen that the gain of the antenna is above 10 dBi over the frequency band 0-4 THz and the radiation efficiency of the antenna is above 80% over the frequency band 0-4 THz.
The normalized radiation patterns of the antenna in four working mode is shown in Fig. 6. By appropriately selecting the driven, director and reflector elements, the antenna can be directed in D 1 (θ = 0 • ), D 2 (θ = 90 • ), D 3 (θ = 180 • ) , and D 4 (θ = 270 • ) directions. The antenna has the capability to reconfigure its radiation direction, covering a 360 • angle at operational frequency 1.25 THz. Figure 7 presents 3D far-field radiation patterns of the proposed antenna in different frequency points ranging from 1 to 3 THz. Similar to the classical Yagi-Uda structure, the present proposed antenna employs a single reflector and more than one director. In the present work, a director and reflector are the graphene strip with chemical potential µ c = 0.4 eV and  www.nature.com/scientificreports/ µ c = 0.8 eV, respectively. This is in contrast to a classical Yagi-Uda antenna structure, in which the parasitic elements get the induced current only from the excited element. Very few graphene-based Yagi-Uda THz antennas are available in the literature. We have compared the performance of the present antenna with graphene-based Yagi-Uda THz antennas reported in the literature, in Table 5. We can notice that the present antenna design has promising result in terms of antenna gain, band width, efficiency and front to back ratio.

Discussion
The major concern in THz communications is the significant path loss effects on account of molecular absorption 33 . As a result, in order to achieve high data transfer rates, THz systems require devices with multiple antennas and beamforming capabilities. The proposed multibeam antennas are expected to meet this requirement. In traditional low-frequency smart antennas, complex signal processing is required to make a decision for the switched-beam antenna as to which beam to access at any time. In contrast, in the proposed THz antennas, the complexity is reduced significantly because only a voltage change is needed for steering the beam direction. The proposed THz antennas can act as promising candidate for THz communication systems due to its orthogonal beam patterns. Recently, intelligent wireless environments have been proposed as promising smart environments that can make capable of sensing and manipulating wavefronts emitted by user devices 34,35 . This kind of environments    The results reveal that the proposed graphene-based THz antenna is promising for THz wireless communication systems. As the future wireless communication system will rely on THz frequency in a big way, the proposed ultra-wideband high gain beam reconfigurable THz antenna structure will cater to the need of the ever-growing wireless communication users.

Method
The CST Studio Suite EM simulator is used to model the graphene and validate the proposed designed antenna 38 .
In the EM simulator, the graphene element is modelled as a thin resistive sheet with surface impedance Z s = 1/σ s , where σ s is the surface conductivity of graphene and depends on angular frequency ω , chemical potential µ c , relaxation time τ and temperature T. The thickness of graphene strip t g = 1 nm is considered 39 . In the present work, room temperature T = 300 K, Fermi velocity v f = 10 6 m/s and mobility of graphene µ g = 10000 cm 2 /Vs are considered 40 . The relaxation time of graphene is computed as τ = µ g µ c /ev 2 f 3 . The discrete face port (fed by a 50 ohm source) is considered for the excitation of the antenna structure in the CST Studio Suite EM simulator. For mode D 1 (θ = 0 • ) and D 3 (θ = 180 • ) , excitation is provided at the centre of driven element 1, whereas for mode D 2 (θ = 90 • ) and D 4 (θ = 270 • ) , excitation is provided at the centre of driven element 8. The performance of the proposed graphene THz antenna model is studied in the wide spectrum from 0 to 4 THz.
In the EM simulator, the changing of chemical potential parameter enables the reconfiguration in the graphene THz antenna. To achieve the different modes in the proposed graphene THz antenna, the chemical potential of graphene elements varies from 0.4 eV to 0.8 eV. The variation of chemical potential is achieved by applying different bias voltages to graphene strips. In the present case, the chemical potential µ c = 0.4 eV is achieved with a bias voltage V b = 1. 81 V, generating a static electric field E = 9.05 kV/cm. With the increase in the applied bias voltage, more charge is induced on the graphene strip, which in turn increases the chemical potential. The bias voltage of 1.8, 2.7, and 3.6 V are required to enable the beam reconfiguration of the proposed antenna structure.
The feasibility of the fabrication of the proposed antenna structure can be explained with the help of Fig. 1b. Mechanical support of graphene-based Yagi-Uda antenna structure can be provided by using a silicon wafer sample. Using the e-gun evaporation technique, a thin polysilicon layer of 100 nm can be evaporated on the silicon wafer sample surface 41 . On top of the polysilicon layer, a silicon dioxide layer of thickness 2 µm can be www.nature.com/scientificreports/ spin-coated and then, it should be heated at 300 °C for one hour. The chemical vapour deposition grown graphene strips can then be transferred onto the silicon dioxide layer. Graphene conductive ink is also promising for printed electronics. The antenna can also be fabricated using graphene ink on the silicon dioxide substrate. The reconfigurable capabilities of the proposed graphene THz antenna structure can be demonstrated experimentally by independent control of each graphene strip through different biasing gates 32,42,43 . The different bias voltages enable the different chemical potential on graphene, leads to control of surface conductivity in graphene sheets. The tunable conductivity nature of the graphene elements in the proposed antenna enables beam reconfigurability. The modulation of graphene conductivity experimentally can also be achieved by a photoexcitation process 41 .